This is the 723rd most frequent Czech word.
polsko
Polsko: Poland (a country in Central Europe).
Here, 'polsko' is a proper noun and serves as the subject of the sentence, referring to the country Poland.
Polsko je známé svými historickými městy.
Poland is known for its historical cities.
Here, 'polsko' is a proper noun in its locative case form 'v Polsku' indicating a location, referring to the country Poland.
V Polsku mají krásné hory a lesy.
In Poland, they have beautiful mountains and forests.
In this sentence, 'polsko' is a proper noun and serves as the direct object, indicating the destination of the visit.